I see. Here is a CSS you may try adding to change the color of the toggle menu:
.main-navigation button.menu-toggle {
color: var(--base);
}
This uses a color from the Global colors in Appearance > Customize > Colors. You may choose a difference Global color by replacing base with the CSS variable name of the Global color you wish to use.
See: https://share.getcloudapp.com/nOuRdN9L
Alternatively, here is a code if you wish to use rgb:
.main-navigation button.menu-toggle {
color: rgb(255, 255, 255);
}
Either should work.
Here is an article with regards to adding CSS: https://docs.generatepress.com/article/adding-css/#additional-css
Adding it through Additional CSS should work.
With regards to uploading screenshots: https://docs.generatepress.com/article/using-the-premium-support-forum/#uploading-screenshots
Hope this helps. 🙂